Vendor supplied encryption programs tend to be not very secure. For something like this, assuming the security is to protect against random crooks rather than against (say) the FBI, moderate security might be ok.

If you're willing to trust moderate security, I'd probably just use password protected zip files with (if you're using wndows) winzip 9.0 or later, if I have it right (the earlier password system was much less secure).

If you want higher security, use PGP/GPG. Include a copy of the program (unencrypted) on the USB key itself, preferably a DOS command line version (that will run on just about any PC). You might even include an image of a bootable MSDOS floppy disc that includes command line PGP. If you have any way to read the USB drive, you can probably find a PC somewhere that can boot the floppy.

Keep in mind that the PC you use it on might not be secure. For example, if you use a PC in a government operated shelter, it might be recording all your keystrokes including your passphrase. Again, depending on the type of content on the USB drive, that might or might not bother you.